This invention relates to a new and improved steering apparatus and more specifically to a steering apparatus in which the displacement of a pump is increased upon initiation of a steering operation.
A known apparatus for varying the displacement of a pump in response to variations in demand for fluid includes a flow control valve. The flow control valve is operable from an intermediate position to either one of two actuated positions to operate a motor to either increase or decrease the displacement of the pump. When the displacement of the pump matches fluid flow requirements, the flow control valve is in the intermediate position and the displacement of the pump is maintained constant. When the demand for fluid increases or decreases, the flow control valve is actuated to change the displacement of the pump. With this known system, the flow control valve must be actuated before the displacement of the pump is increased.
A known apparatus for controlling the operation of a steering motor to turn a steerable vehicle wheel is disclosed in U.S. patent application Ser. No. 243,497 filed Mar. 13, 1981 by Jim L. Rau and Ronald L. LaHue and entitled "Hydrostatic Load-Sense Steering System". This application discloses a priority valve which responds to changes in the demand for fluid by a steering controller. Upon initiation of a steering operation, a pilot flow of fluid is restricted. The pilot fluid flow restriction causes a back pressure to develop in a pilot fluid flow line. This back pressure acts on a priority valve spool to move the spool into a position to direct an increased fluid flow to the steering controller. Although the steering system shown in the aforementioned application is generally satisfactory in its mode of operation and is effective to respond quickly to changes in the demand for fluid by the power steering system, the priority valve must be actuated before the demand for steering fluid is satisfied.
When a variable displacement pump is used in a power steering system, the displacement of the pump must be quickly increased upon initiation of a steering operation in order to satisfy the impending demand for power steering fluid. If increasing the displacement of the pump is delayed until after a flow control valve is actuated, the pump may not be able to satisfy the demand for power steering fluid during an initial portion of a steering operation.